For producing corrugated paperboard, so-called single facers are used that, through an arrangement of corrugating rollers and gluing members, convert two smooth paper sheets into a single face corrugated paperboard, formed by a fluted paper sheet and a smooth paper sheet, also called liner.
For producing a continuous single face corrugated paperboard, two continuous paper sheets are fed to the single facer. The first sheet is fluted by means of a pair of corrugating rollers, each of which is provided with flutes or teeth and meshing with each other at said teeth or flutes in a corrugating nip. The first sheet is corrugated by passing through the corrugating nip. Downstream of the corrugating nip, a glue applicator applies glue to the crests of the flutes formed on the corrugated first paper sheet. Once glue has been applied, the first paper sheet is bonded to the smooth second paper sheet, exerting a sufficient pressure by means of a pressing member, usually a roller, pressing against one of the corrugating rollers and forming a pressure or gluing nip therebetween. In this way, the so-called single-face corrugated paperboard is obtained, which is formed by the corrugated sheet bonded to the smooth sheet or liner.
One of the critical aspects in the operation of single facers is that the corrugated paper sheet tends to detach from the corrugating roller in the path comprised between the corrugating nip and the pressure nip.
Various systems have been studied to reduce the risk of detachment of the corrugated paper sheet from the corrugating roller.
For example, GB 2170831 discloses a system, wherein an arched comb is arranged around the second corrugating roller, the comb extending from the first corrugating roller to the pressure roller and exerting a radial retention action, from the outside inwards, on the corrugated paper sheet in order to keep it adhering to the second corrugating roller. This system has many drawbacks, in particular in case of breakage of the paper sheet.
EP 0092079 discloses a single facer wherein, to keep the corrugated paper sheet adhering to the second corrugating roller, a pressure system is used that, through pressurized air, applies a pushing action against the outer surface of the corrugated paper sheet. The generated airflow causes drawbacks. For example, it tends to draw glue residues that are thus dispersed in the surrounding environment.
U.S. Pat. No. 4,368,094 discloses a single facer wherein, to keep paper adhering to the second corrugating roller in the path between the corrugating nip and the pressure nip, annular grooves in the second corrugating roller are depressurized through suction ducts. The system is particularly complex and entails high thermal losses due to the need of sucking a great quantity of air in the area surrounding the corrugating roller. This latter is heated to facilitate corrugating and gluing. The presence of suction ducts results in a loss of thermal energy from the corrugating rollers and therefore negatively affects the energy consumption of the machine.
A single facer with a different suction system is disclosed in U.S. Pat. No. 4,251,313.
U.S. Pat. No. 4,581,095 discloses a single facer where two suction boxes are associated with the second corrugating roller, one adjacent to the corrugating nip and the other adjacent to the pressure nip, generating a suction in annular grooves provided in the second corrugating roller in order to keep the corrugated paper sheet on the surface of the second corrugating roller. In this case again, even if sealing systems are provided, there is a high air consumption with consequent heat removal from the corrugating roller. Moreover, the detachment of the single face corrugated paperboard from the corrugating roller is particularly difficult and there are frequent breakages of the single face corrugated paperboard, with consequent jam of the single facer.
U.S. Pat. No. 4,618,394 discloses a single facer comprising a first corrugating roller with a first series of flutes or teeth and a second corrugating roller with a second series of flutes or teeth. The two series of flutes of the two corrugating rollers mesh with one another in a corrugating nip. The second corrugating roller comprises a series of annular grooves. A pressing roller co-acts with the second corrugating roller, defining a pressure nip where the corrugated first paper sheet is glued to the smooth second paper sheet. In this case, in order to keep the corrugated first paper sheet adhering to the surface of the second corrugating roller, a suction box is provided, extending approximately parallel to the axis of the second corrugating roller and defining a suction area around the second corrugating roller. The suction box is arranged adjacent the corrugating nip, downstream of the pressing roller and upstream of the first corrugating roller with respect to the direction of rotation of the second corrugating roller. The suction box generates a suction in the annular grooves, keeping the first paper sheet adhering to the second corrugating roller. The single face corrugated paperboard is detached from the second corrugating roller at the pressure nip by means of foils entering in the annular grooves and arranged between the suction box and the pressing roller. The foils are distanced from the suction box and extend under the corrugated paperboard up to the nip between the second corrugating roller and the pressing roller. These also limit the area where the suction is generated through the suction box.
This known system still has drawbacks, in particular due to the air consumption and, thus, to the consequent thermal losses. Moreover, the conditions under which the corrugated paper sheet and the smooth paper sheet are glued together are not optimal. Also detaching the single face corrugated paperboard from the corrugating roller downstream of the pressure or gluing nip is difficult.
A need therefore exists for further improving the single facers to alleviate or overcome, the drawbacks of the current art.